1. Field of the Invention
The present invention relates generally to improvements in oil well pumping units and more particularly, but not by way of limitation, to improvements in pump jacks used to operate downhole pumps connected to a horsehead of the pump jack via a polish rod and sucker rods extending down the well to the pump.
2. Description of the Prior Art
A common system for pumping an oil well is to provide a pump jack on the earth's surface, adjacent the well, to operate a downhole pump; that is, a pump that is located in the well and connected to the pump jack via a plurality of sucker rods extending upwardly to a polish rod that is connected to the pump jack. In a system of this type, the pump jack includes a walking beam that is pivotally mounted atop a samson post, forming a part of a pump jack base, and the walking beam is oscillated in a vertical arc to alternately raise and lower a horsehead mounted on one end of the walking beam. The polish rod is connected to the horehead so that the oscillation of the walking beam is translated into a reciprocation of the polish rod that is transmitted to the pump via the sucker rods to operate the pump.
A variety of drive systems, each having its own advantages and disadvantages, can be used to oscillate the walking beam. In many respects, a particularly advantageous system is one in which a hydraulic actuating cylinder is connected between the walking beam and the pump jack base so that the piston rod of the hydraulic actuating cylinder can be alternately extended and retracted, under the control of a suitable hydraulic circuit, to oscillate the walking beam. As has been discussed in my aforementioned co-pending application, a major advantage of such a system is that it offers possibilities for easily adjusting the stroke and reciprocation rate of the polish rod, to meet existing conditions at a well, that cannot be matched with other types of systems for oscillating the walking beam. However, such pump jack systems are not without problems. In particular, for efficient and substantially trouble-free operation of a pumping system using a pump jack, it is desirable that the walking beam of the pump jack undergo motion that is substantially harmonic; that is, motion in which the speed of the walking beam slowly decreases, and subsequently slowly increases in an opposite direction, near the end points of its arc of travel while the walking beam moves at a relative high speed through the center of such arc. A problem which has not been solved in prior art pump jacks is that of causing the walking beam of a pump jack utilizing a hydraulic actuating cylinder drive system to undergo such motion, especially without unduly increasing the cost of construction of the pump jack, or forfeiting other advantages that pump jacks of this type can have, or both. The present invention solves this problem.